In a conventional type 301 lockstitch sewing machine, one of the major difficulties is in the timing relationships existent between the needle thread take-up, the loop taker operation for stitch concatenation and the feed timing and duration, each of which take time in the machine cycle. Needle thread take-up, loop formation, loop expansion, loop extraction and feed each require separate and distinct portions of the machine cycle, with preferably no overlap. For this reason, loop seizure by the loop taker has to be coordinated with the needle bar and the take-up and loop extraction has to be coordinated with cast off of the loop from the loop taker. The feed time must also be related to the take-up, it must start at a time when the feed dog is up and must continue beyond the time the take-up is up. The handling of the needle thread is also critical in that the thread demand for loop formation and for expansion by the loop taker must be coordinated with the thread supply from the take-up.
Most current commercial rotary hook lockstitch sewing machines use a two to one ratio of loop taker to arm shaft rotation so that the loop taker revolves twice for every complete cycle of the needle bar. Cast off of the needle thread from the loop taker following concatenation of the needle thread around the bobbin thread occurs after almost 270 degrees of machine arm shaft rotation from the "needle up" position. The time from loop seizure to cast off takes about 110 degrees of arm shaft rotation. A ratio of loop taker to arm shaft rotation of three to one, however, enables the machine to approach cast off in only 70 degrees of machine arm shaft rotation after loop seizure instead of 110 degrees, a gain of 35 degrees that can be used, for example, to complete feeding earlier in a machine cycle. Since the feed time is related to the needle timing, the net effect is a gain in capacity to sew heavy materials, the difference being from a capacity of about 1/16 inch material in a conventional consumer type sewing machine having a two-to-one hook to arm shaft ratio, to a capacity of about 1/4 inch thick material using a three-to-one ratio. That is, before the needle reaches the material, even if the material is 1/4 inch thick, in a three to one ratio machine the feed is completed. This potential for increase has been previously recognized and is discussed, for example, in Hemleb, U.S. Pat. No. 1,583,925 dated May 11, 1926.
In advancing the feed, needle bar drive and take-up relative the loop taker, however, the take-up also starts its descent earlier and pays out needle thread faster than the needle bar demand, providing slack thread. If this slack thread is not controlled prior to needle penetration, the thread can wrap around the needle point, prevent loop siezure, break the thread, or interfere with correct stitch formation. Following needle penetration, however, substantial needle thread needs to be subsequently metered out for loop seizure and expansion to pass around the bobbin. Various prior art 3 to 1 machines used various methods to control the needle thread, such as complex rotary take-ups or lengthened needle bars, each of which required substantial changes from the currently prefered crank driven take-up design.